Colloidal semiconductor nanoparticles, named “quantum dots” (QDs), are crystalline objects that exhibit specific fluorescence properties. Their absorption cross section is very large, they are bright and their emission spectra have a small full width half maximum, and a peak wavelength that is tunable as a function of their composition, their size and their shape (in the range of a few nanometers to few tens of nanometers). They are also far more resistant to photobleaching than traditional organic dyes. These unique features make them very attractive for diverse applications in the field of medical and biological imaging, such as individual proteins monitoring, multi-color immunostaining, stem cells tracking, fluorescence acquisition cell sorting, or optically assisted surgery.
Functionalization of QDs surface presents many advantages. Especially, a second imaging agent may be coupled at the surface of the QD, to enable bimodal imaging. On another hand, functionalizing QDs' surface by coupling bio-targeting moieties is interesting for bio-imaging. Functionalization by a therapeutically active molecule may also be interesting.
QD syntheses provide colloidal solutions of fluorescent nanocrystals capped with ligands. Functionalization of QD may thus be performed by functionalizing ligands presents at their surface.
Typical QD syntheses provide QDs capped with hydrophobic ligands, while the use of QDs in live-cell imaging requires their complete solubility in water as well as an excellent compatibility with biological media. To make the QDs water-soluble, one method is to perform cap exchange, consisting in the replacement of original ligands by hydrophilic ones, bearing a chemical function able to bind to the nanocrystal surface (Chan et al. Science 1998, 281:2016 and Mattoussi et al. J. Am. Chem. Soc. 2000, 122:12142). Cap exchange results in small and stable QDs. The non-specific interactions of the QD with cell membranes or with biomolecules in general depend mainly on the moieties that are adsorbed on the QD surface. Ligand exchange provides a versatile method to control the size, the nature of the ligand as well as its affinity for the QD surface (ligands that are too strong can indeed dissolve the QD, while ligands that are not strong enough can detach from the QD surface).
Ligand desorption is a strong limitation for the use of QDs in bio-imaging. This desorption, favored in high diluted conditions, causes indeed a loss of colloidal stability and functionality, as well as an increase in aggregation and non-specific adsorption. As a consequence, continued efforts have been made to improve the affinity of passivating ligands for the QD surface. The design of these replacing ligands is also guided by further needs for biological applications of quantum dots, namely: small size; stability over a large pH range, at elevated salt concentrations and in a cellular medium; low non-specific adsorption; and possible functionalization afterwards.
To match the above-mentioned criteria, especially limitation of ligand desorption and provision of small and stable QDs, the Applicant developed a multidentate zwitterionic ligand L2, issued from the copolymerization of a bidentate monomer and a monozwitterionic monomer (Scheme 1) (Giovanelli et al. Langmuir 2012, 28, 15177-15184).

The structure of the lateral chains of L2 was evidenced to enhance affinity of this ligand toward QDs and thus avoid ligand desorption while keeping a small size of particle.
Derivatives of L2, L2-NH2 and L2-PEG-NH2 were also proposed to introduce functionalizable amine later chains (scheme 2):

Especially, QD functionalization was performed for L2-NH2 by conjugation with fluorescein. L2-PEG-NH2-capped QDs were also conjugated via their amine function, with either biotin or avidin by peptidic coupling after capping QD.
Therefore, the Applicant provided a first generation of hydrophilic ligands having a very good affinity for QD and being functionalizable. However, with this first generation of ligands, functionalization requires to use at least three different monomers to form the ligand: an anchoring monomer, an hydrophilic monomer and a functionalizable monomer. Therefore, the resulting ligand is quite complex. Moreover, it was evidenced that functionalization of this type of ligand modifies its anchoring properties and therefore modifies the stability of the QD.
Therefore, there is a need for more simple ligands which remain functionalizable and for which functionalization does not modify anchoring properties.
The Applicant surprisingly showed that anchoring moieties of L2 ligand may be labile. If there is a sufficient number of anchoring functions in the ligand, a part of them is not linked to the surface of the QD and they may be used for functionalizing the ligand, without decreasing the anchoring stability. This presents the advantage of not requiring a modification of the ligand by a third functionalizable monomer. There is only the need to control the length of the ligand to ensure that there is a sufficient number of anchoring monomers to achieved both anchoring and functionalization.
Moreover, the Applicant evidenced that the ligand of the invention, which may be synthesized by radical polymerization in presence of CTA (chain transfer agent), bears one functionalizable end. Indeed, it was surprisingly shown that upon CTA polymerization, ligands bearing at the first end a hydrogen atom and at the other end the CTA were predominantly obtained, whereas numerous possibilities were expected for ends of the polymeric chain.
By varying the CTA to obtain a functionalizable end of the ligand and by selecting the anchoring moiety of the anchoring monomer, orthogonal functionalization of the ligand may be envisaged to introduce 2 types of functional groups on the ligand and thus on the QD. In this case, a first functional group may be introduced at one end of the ligand, while a second functional group may be introduced on some on the anchoring moieties on lateral chains.
Therefore, the Applicant herein provides a new ligand of QDs, which is a copolymer obtained from at least 2 monomers, said monomers being:                one anchoring monomer A having a side-chain comprising a first moiety MA having affinity for the surface of a nanocrystal, and        one hydrophilic monomer B having a side-chain comprising a second moiety MB being hydrophilic,wherein one end of the copolymer is a hydrogen atom and the other end comprises a functionalizable group.        
The functionalizable ligand of the invention presents the advantage that even coated on QDs, functionalizable end groups of the polymer are accessible to conjugation. Therefore, the ligand may be functionalized after complexation of QD.